build llvm libc

[nickdesaulniers]

Follow up to #36 (comment).
Specifically, it would be nice to test out memcpy, memset, memcmp, and strlen from llvm-libc.

There's a few issues though. We'd maybe want to build it as part of epoch2, but:

1. llvmlibc has some build failures in 14.0.1. I need to cherry-pick a hacked up version of commit 122da690b39a ("[libc] Fix build when __FE_DENORM is defined").
2. llvmlibc build seems to depend on musl-dev to build in the env (wtf)
3. llvmlibc build seems to depend on kernel headers, which we don't build for epoch2 but do for epoch3. I think we might need to thus build llvmlibc BEFORE building clang in epoch3...

Alternatively, clang can assemble some glibc implementations of the above. 😈

cc @compnerd

[nickdesaulniers]

Here's a profile of llvmlibc used in stage3 to build a kernel:

Samples: 12M of event 'cycles:ppu', Event count (approx.): 9050605813458
  Overhead  Shared Object             Symbol
+    3.04%  clang                     [.] clang::SourceManager::getFileIDLocal
+    2.12%  clang                     [.] llvm::StringMapImpl::LookupBucketFor
+    1.68%  clang                     [.] clang::TokenLexer::Lex
+    1.64%  clang                     [.] clang::Lexer::LexTokenInternal
+    1.06%  clang                     [.] clang::Preprocessor::Lex
+    1.04%  clang                     [.] clang::ASTContext::getDeclAttrs
+    1.03%  clang                     [.] clang::Lexer::LexIdentifierContinue
+    0.99%  clang                     [.] GetFullTypeForDeclarator
+    0.97%  clang                     [.] (anonymous namespace)::IntExprEvaluator::VisitBinaryOper
+    0.94%  clang                     [.] CheckICE
+    0.81%  clang                     [.] clang::TokenLexer::ExpandFunctionArguments
+    0.81%  clang                     [.] clang::SourceManager::getFileID
+    0.66%  clang                     [.] memcpy
     0.58%  clang                     [.] free
+    0.54%  clang                     [.] clang::Parser::ParseDeclarationSpecifiers
     0.52%  clang                     [.] strlen
+    0.51%  clang                     [.] clang::Preprocessor::getMacroDefinition
     0.50%  clang                     [.] GetDiagInfo
     0.49%  clang                     [.] llvm::FoldingSetBase::FindNodeOrInsertPos
     0.47%  clang                     [.] memset
     0.47%  clang                     [.] AnalyzeImplicitConversions
     0.46%  clang                     [.] clang::Sema::LookupName
     0.43%  clang                     [.] malloc
     0.43%  clang                     [.] clang::Parser::ParseCastExpression
     0.43%  clang                     [.] clang::DiagnosticIDs::getDiagnosticSeverity
     0.42%  clang                     [.] clang::Preprocessor::ReadMacroCallArgumentList
     0.41%  clang                     [.] clang::Expr::IgnoreParenImpCasts
     0.40%  clang                     [.] clang::RecursiveASTVisitor<(anonymous namespace)::Fallth
     0.40%  clang                     [.] clang::Sema::ActOnFunctionDeclarator
     0.39%  clang                     [.] memcmp

compare vs musl. Or:

musl:

+    1.40%  clang                      [.] memcpy
+    0.65%  clang                      [.] memset
     0.50%  clang                      [.] memcmp
     0.46%  clang                      [.] strlen

llvm-libc:

+    0.66%  clang                     [.] memcpy
     0.52%  clang                     [.] strlen
     0.47%  clang                     [.] memset
     0.39%  clang                     [.] memcmp

(memcmp falls out of the top 20)
so llvm-libc is a win here (maybe not for strlen though :( )