xtool/safesplit:properly handle consecutive short flags

[luoliwoshang]

fix #1244

at the homebrew test case will set the CFLAGS

==> CFLAGS: -Os -w -pipe  -mmacosx-version-min=15 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk

/opt/homebrew/Cellar/llgo/0.12.12/libexec/crosscompile/clang/bin/clang++ -Os -w-pipe -mmacosx-version-min=15 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk -Qunused-arguments -Wno-unused-command-line-argument --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -I/opt/homebrew/Cellar/llgo/0.12.12/libexec/crosscompile/clang/include -o /Users/zhangzhiyang/Library/Caches/Homebrew/go_cache/9f/9ff6c578d2686798b95abb59ea500cd2091450c2073cd84e1ab7d4bacfedafb0-d.o -c /private/tmp/llgo-2984895133.ll -Wno-override-module

clang++: error: unknown argument '-w-pipe'; did you mean '--pipe'?
panic: export object of internal/goarch failed: exit status 1

the -w-pipe is from -w and -pipe, current process combine them to -w-pipe is not expected

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.80%. Comparing base (cf2d1ef) to head (0935d10).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1243   +/-   ##
=======================================
  Coverage   87.80%   87.80%           
=======================================
  Files          35       35           
  Lines        8941     8941           
=======================================
  Hits         7851     7851           
  Misses        979      979           
  Partials      111      111           

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