feat(asm): PCALIGN alignment on amd64

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -22,13 +22,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   relocation, per-GOOS); arm64 accepts the bare-register indirect branch
   (`BL R9` beside `BL (R9)`, both BLR) and the zero-immediate store
   (`MOVD $0, mem` through the zero register, rejecting non-zero immediates
-  as the toolchain does); and `gasm asm` predefines the `GOARCH_<arch>`
-  and `GOOS_<goos>` macros the go command passes to `go tool asm`, so
-  GOROOT headers' `#ifdef GOARCH_amd64` platform blocks (`go_tls.h`'s
-  `get_tls` and friends) select as intended.  The GOROOT corpus measure
-  moves to 276 of 353 files assembling for every target architecture
-  (78.2 %), 91.4 % of the real-code corpus (280 of 303), from 70.8 %
-  and 82.2 %.
+  as the toolchain does); `PCALIGN` now aligns on amd64, padding with the
+  toolchain's greedy single-instruction NOPs; and `gasm asm` predefines
+  the `GOARCH_<arch>` and `GOOS_<goos>` macros the go command passes to
+  `go tool asm`, so GOROOT headers' `#ifdef GOARCH_amd64` platform blocks
+  (`go_tls.h`'s `get_tls` and friends) select as intended.  The GOROOT
+  corpus measure moves to 285 of 353 files assembling for every target
+  architecture (80.7 %), 91.4 % of the real-code corpus (280 of 303),
+  from 70.8 % and 82.2 %.
 
 ### Added