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 linked.

This issue is found while building OpenSBI with 'H' extension (resulting
ELF files did not contain "h1p0" in "Tag_RISCV_arch").

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.

ld/ChangeLog:

	* testsuite/ld-riscv-elf/attr-merge-arch-02.d: Test merging of 'H'
	and 'Zicsr' extensions.
	* testsuite/ld-riscv-elf/attr-merge-arch-02a.s: Likewise.

bfd/elfnn-riscv.c

@@ -3427,7 +3427,7 @@ riscv_merge_std_ext (bfd *ibfd,
 		     struct riscv_subset_t **pin,
 		     struct riscv_subset_t **pout)
 {
-  const char *standard_exts = "mafdqlcbjtpvn";
+  const char *standard_exts = "mafdqlcbjtpvnh";
   const char *p;
   struct riscv_subset_t *in = *pin;
   struct riscv_subset_t *out = *pout;

ld/testsuite/ld-riscv-elf/attr-merge-arch-02.d

@@ -6,4 +6,4 @@
 
 Attribute Section: riscv
 File Attributes
-  Tag_RISCV_arch: "rv32i2p1_a2p0"
+  Tag_RISCV_arch: "rv32i2p1_a2p0_h1p0_zicsr2p0"

ld/testsuite/ld-riscv-elf/attr-merge-arch-02a.s

@@ -1 +1 @@
-	.attribute arch, "rv32i2p1_a2p0"
+	.attribute arch, "rv32i2p1_a2p0_h1p0_zicsr2p0"