aarch64: Remove AARCH64_FL_RCPC8_4 [PR107025]

aarch64: Remove AARCH64_FL_RCPC8_4 [PR107025]

AARCH64_FL_RCPC8_4 is an odd-one-out in that it has no associated
entry in aarch64-option-extensions.def.  This means that, although
it is internally separated from AARCH64_FL_V8_4A, there is no
mechanism for turning it on and off individually, independently
of armv8.4-a.

The only place that the flag was used independently was in the
entry for thunderx3t110, which enabled it alongside V8_3A.
As noted in PR107025, this means that any use of the extension
will fail to assemble.

In the PR trail, Andrew suggested removing the core entry.
That might be best long-term, but since the barrier for removing
command-line options without a deprecation period is very high,
this patch instead just drops the flag from the core entry.
We'll still produce correct code.

gcc/
	PR target/107025
	* config/aarch64/aarch64.h (oAARCH64_FL_RCPC8_4): Delete.
	(AARCH64_FL_FOR_V8_4A): Update accordingly.
	(AARCH64_ISA_RCPC8_4): Use AARCH64_FL_V8_4A directly.
	* config/aarch64/aarch64-cores.def (thunderx3t110): Remove
	AARCH64_FL_RCPC8_4.