xtensa: reserve relocation number for R_XTENSA_32_ABS

R_XTENSA_32 uses a partial_inplace howto, so BFD stores its addend in
the relocated word rather than in r_addend, which deviates from the
generic ABI for an SHT_RELA target.  R_XTENSA_32_ABS is the same 32-bit
absolute address computed as S + A, with the addend taken only from
r_addend.

Reserve its number ahead of the implementation so that other toolchains
can rely on it while the rest of the change is under review.

include/
	* elf/xtensa.h (R_XTENSA_32_ABS): New relocation, number 64.