Added TX_ENABLE_FIQ_SUPPORT to the feature-macro assembly stage

[fdesbiens]

#608 assembled the code behind TX_ENABLE_VFP_SUPPORT, TX_LOW_POWER and
TX_ENABLE_EXECUTION_CHANGE_NOTIFY, and missed TX_ENABLE_FIQ_SUPPORT, which
guards assembly in 145 files across the A and R profile ports.

All 145 assemble today, so this adds no fix — only the regression protection the
other three already have. It is the same argument as #608: a guarded path that
nothing assembles is a path where the next .arch-class mistake waits.

It also records why TX_ENABLE_IRQ_NESTING and TX_ENABLE_FIQ_NESTING are not
in the list, since their absence otherwise reads as the same oversight I am fixing
here. They guard no assembly in the trees this script walks: the nesting start and
end routines are separate files compiled unconditionally, and the macros only feed
the TX_PORT_SPECIFIC_BUILD_OPTIONS bitfield in tx_port.h. Adding them would
assemble nothing new while implying coverage that does not exist.

Verified with Arm Toolchain for Embedded 22.1.0:

== Assembly sources of every Arm gnu port ==
  711 of 711 assembled
== Assembly behind feature macros ==
  TX_ENABLE_VFP_SUPPORT: 37 of 37 assembled
  TX_ENABLE_FIQ_SUPPORT: 145 of 145 assembled
  TX_LOW_POWER: 8 of 8 assembled
  TX_ENABLE_EXECUTION_CHANGE_NOTIFY: 218 of 218 assembled