Remove "slow target" flag for `mips(el)-linux` targets in tests with LLVM 20

[alexrp]

Right now, we're flagging these targets as slow by default, causing them to not get run in tests unless -Dtest-slow-targets is passed.

zig/test/tests.zig

Lines 313 to 365 in 11176d2

	.{
	.target = .{
	.cpu_arch = .mips,
	.os_tag = .linux,
	.abi = .none,
	},
	.slow_backend = true,
	},
	.{
	.target = .{
	.cpu_arch = .mips,
	.os_tag = .linux,
	.abi = .musl,
	},
	.link_libc = true,
	.slow_backend = true,
	},
	.{
	.target = .{
	.cpu_arch = .mips,
	.os_tag = .linux,
	.abi = .gnueabihf,
	},
	.link_libc = true,
	.slow_backend = true,
	},
	.{
	.target = .{
	.cpu_arch = .mipsel,
	.os_tag = .linux,
	.abi = .none,
	},
	.slow_backend = true,
	},
	.{
	.target = .{
	.cpu_arch = .mipsel,
	.os_tag = .linux,
	.abi = .musl,
	},
	.link_libc = true,
	.slow_backend = true,
	},
	.{
	.target = .{
	.cpu_arch = .mipsel,
	.os_tag = .linux,
	.abi = .gnueabihf,
	},
	.link_libc = true,
	.slow_backend = true,
	},

We do this because it takes 30-60 minutes to compile the tests for these targets due to a severe LLVM performance issue. The upstream issue is filed here: llvm/llvm-project#104562

This issue is to track removing the "slow target" flag from these if/when the LLVM bug gets fixed.

[alexrp]

Bad news: #21125

Good news: llvm/llvm-project#104723

[alexrp]

The LLVM 19 backport for this was rejected, so this will have to wait until LLVM 20.