libgo: add 32-bit RISC-V (RV32) support

Add support for the 32-bit RISC-V (RV32) ISA matching the 64-bit RISC-V
(RV64) port except for async preemption added as a stub only.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/251179
diff --git a/libgo/match.sh b/libgo/match.sh
index cd35942..6f7b368 100755
--- a/libgo/match.sh
+++ b/libgo/match.sh
@@ -116,7 +116,7 @@
 	aix | android | darwin | dragonfly | freebsd | illumos | hurd | js | linux | nacl | netbsd | openbsd | plan9 | solaris | windows)
 	    tag1=nonmatchingtag
 	    ;;
-	386 | amd64 | amd64p32 | arm | armbe | arm64 | arm64be | alpha | ia64 | m68k | mips | mipsle | mips64 | mips64le | mips64p32 | mips64p32le | nios2 | ppc | ppc64 | ppc64le | riscv64 | s390 | s390x | sh | shbe | sparc | sparc64 | wasm)
+	386 | amd64 | amd64p32 | arm | armbe | arm64 | arm64be | alpha | ia64 | m68k | mips | mipsle | mips64 | mips64le | mips64p32 | mips64p32le | nios2 | ppc | ppc64 | ppc64le | riscv | riscv64 | s390 | s390x | sh | shbe | sparc | sparc64 | wasm)
 	    tag1=nonmatchingtag
 	    ;;
     esac
@@ -128,7 +128,7 @@
 	aix | android | darwin | dragonfly | freebsd | hurd | illumos | js | linux | nacl | netbsd | openbsd | plan9 | solaris | windows)
 	    tag2=nonmatchingtag
 	    ;;
-	386 | amd64 | amd64p32 | arm | armbe | arm64 | arm64be | alpha | ia64 | m68k | mips | mipsle | mips64 | mips64le | mips64p32 | mips64p32le | nios2 | ppc | ppc64 | ppc64le | riscv64 | s390 | s390x | sh | shbe | sparc | sparc64 | wasm)
+	386 | amd64 | amd64p32 | arm | armbe | arm64 | arm64be | alpha | ia64 | m68k | mips | mipsle | mips64 | mips64le | mips64p32 | mips64p32le | nios2 | ppc | ppc64 | ppc64le | riscv | riscv64 | s390 | s390x | sh | shbe | sparc | sparc64 | wasm)
 	    tag2=nonmatchingtag
 	    ;;
     esac