GN: make SkPMColor BGRA on Linux.  All other platforms are auto-detected.
GN: make SkPMColor BGRA on Linux.  All other platforms are auto-detected.

This technically shouldn't cause us to draw differently, but sometimes
does on buggy GMs, and also has us go down slightly different code paths.
It's good to be consistent with GYP, Chromium, Google3, etc.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2214023002

Review-Url: https://codereview.chromium.org/2214023002

HalCanary/skia-hc,google/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,google/skia,rubenvb/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia