fix(cmd): update asm help text to list arm64 as supported

Assisted-by: MiMo V2.5 Pro

cmd/gasm/main.go

@@ -398,8 +398,9 @@ func cmdAsm(args []string) int {
 	fs := newCommand("asm", "gasm asm [--format raw|elf|goobj] [-p pkg] [-o out] <file>", `
 	Assemble FILE without the Go toolchain: every TEXT function is encoded to
 	machine code and printed as a hex dump.  Supported architectures: amd64
-	(including VEX/AVX2 and EVEX/AVX-512), riscv64 (RV64IMAFDC + RVC) and
-	loong64 (LoongArch base ISA); arm64 encoding is not yet implemented.
+	(including VEX/AVX2 and EVEX/AVX-512), arm64 (AArch64 integer, FP,
+	conditional select, CRC32, and MOV pseudo), riscv64 (RV64IMAFDC + RVC)
+	and loong64 (LoongArch base ISA).
 
 With -o the output is written to a file instead.  The --format flag selects
 what is written: raw (the default) concatenates the functions and the data