overlays: consolidate Abseil BMI2 workaround under patches

Extract the header fix into a standalone patch and colocate the
Electron, Deno, and Signal overrides in patches/abseil. Preserve the
x86-64-v3 restriction and document the accepted upstream fix while
waiting for bundled dependency updates.

Import the Abseil overrides through the patches overlay and register
the torchcodec override. Leave PostgreSQL output checks in
x86-64-v3-workarounds.nix.

overlays/patches/abseil/bmi2-public-header.patch

@@ -0,0 +1,20 @@
+Subject: [PATCH] abseil: include BMI2 intrinsics through the public header
+
+GCC and Clang reject direct inclusion of bmi2intrin.h. Include immintrin.h
+instead so that the compiler supplies the required intrinsic setup when
+BMI2 is enabled, including builds targeting x86-64-v3.
+
+This patch is shared by the vendored Abseil copies in Electron, rusty_v8
+(Deno), and Signal's WebRTC build.
+
+--- a/third_party/abseil-cpp/absl/container/internal/raw_hash_set.h
++++ b/third_party/abseil-cpp/absl/container/internal/raw_hash_set.h
+@@ -226,7 +226,7 @@
+ #endif
+ 
+ #ifdef __BMI2__
+-#include <bmi2intrin.h>
++#include <immintrin.h>
+ #endif  // __BMI2__
+ 
+ namespace absl {