build: increase max_rss to 7.8G

[kcbanner]

I've been consistently observing numbers in the 7659786240 ballpark when building with these settings:

e:\dev\zig-windows-x86_64-0.14.0-dev.2034+56996a280\zig.exe build %* ^
  -p build-stage4-release ^
  --search-prefix e:\dev\zig-bootstrap\out-win\x86_64-windows-gnu-native ^
  --zig-lib-dir c:\cygwin64\home\kcbanner\kit\zig\lib ^
  -Dstatic-llvm ^
  -Doptimize=ReleaseSafe ^
  -Dlog=true ^
  -Dstrip=false ^
  -Duse-zig-libcxx ^
  -Dno-langref ^
  -Dtarget=x86_64-windows-gnu