[arm] Rewrite t-rmprofile multilib specification

This is the R- & M-profile equivalent of the previous A-profile
multilib rewrite.  Additionally this patch adds some top-level rules
to help find suitable multilibs for general cases when certain
libraries are not built, or when building for legacy cores.

gcc:

	* config/arm/t-aprofile (v7_a_nosimd_variants, v7_a_simd_variants)
	(v7ve_nosimd_variatns, v7ve_vfpv3_simd_variants)
	(v7ve_vfpv4_simd_variants, v8_a_nosimd_variants, v8_a_simd_variants)
	(v8_1_a_simd_variants, v8_2_a_simd_variants): Move to ...
	* config/arm/t-multilib: ... here.
	(MULTILIB_OPTIONS): Add armv7 and armv7+fp architectures.
	(MULTILIB_MATCHES): Use armv7 libraries for armv7-r.  Also use for
	armv7-a and armv8*-a when A-profile libraries have not been built.
	* config/arm/t-rmprofile: Rewrite.

gcc/testsuite:
	* gcc.target/arm/multilib.exp (rmprofile): New tests when rm-profile
	multilibs have been built.

From-SVN: r249300