RISC-V: Allow merging 'H' extension

RISC-V: Allow merging 'H' extension

Because riscv_merge_std_ext function did not merge the 'H' extension, linked
executables lacked 'H' extension when multiple objects are merged.

This issue is found while building OpenSBI with 'H' extension (resulting
ELF files did not contain "h1p0" in "Tag_RISCV_arch" even if *all* linked
object files contained it).

This commit adds 'h' to standard_exts variable to merge 'H' extension.

bfd/ChangeLog:

	* elfnn-riscv.c (riscv_merge_std_ext): Add 'H' extension merging.