Fix performance issue in misaligned strcpy.
This patch adds new function libc_feholdsetround_noex_aarch64_ctx, enabling
further optimization. libc_feholdsetround_aarch64_ctx now only needs to
read the FPCR in the typical case, avoiding a redundant FPSR read.
Performance results show a good improvement (5-10% on sin()) on cores with
expensive FPCR/FPSR instructions.