sysroot installer: Don't error out for unknown target archs

V8 has several target_archs that don't have a supported
sysroot image, e.g. mips (big endian), ppc, s390.

This prevents the script from failing when run as a hook and
when one of these architectures is used.

BUG=chromium:616032

Review-Url: https://codereview.chromium.org/2027873004
Cr-Commit-Position: refs/heads/master@{#397129}

build/linux/sysroot_scripts/install-sysroot.py

@@ -107,8 +107,6 @@ def DetectTargetArch():
     return 'arm64'
   elif target_arch == 'mipsel':
     return 'mips'
-  elif target_arch:
-    raise Error('Unrecognized target_arch: %s' % target_arch)
 
   return None