Harcoding arch to 'pentium4' prevents building for 64bit Linux.

To reproduce

in the viewer source checkout:

1. Configure for 64bit, e.g.

   autobuild configure -c DebugOS -- -DWORD_SIZE=64

2. Try to build, e.g.

   autobuild build -c DebugOS --no-configure -- -j1

Expected

For non-standalone builds:

• Object files are compiled
• Build fails at link time due to prebuilts not matching the architecture

For 'standalone' builds (append -DSTANDALONE=ON to the configuration command):

• Build succeeds

Observed

(same for standalone and non-standalone)

Scanning dependencies of target cmake

[  0%] Building CXX object cmake/CMakeFiles/cmake.dir/cmake_dummy.o

${SRC_DIR}/indra/cmake/cmake_dummy.cpp:1: error: CPU you selected does not support x86-64 instruction set

${SRC_DIR}/indra/cmake/cmake_dummy.cpp:1: error: CPU you selected does not support x86-64 instruction set

make[2]: *** [cmake/CMakeFiles/cmake.dir/cmake_dummy.o] Error 1

make[1]: *** [cmake/CMakeFiles/cmake.dir/all] Error 2

make: *** [all] Error 2

ERROR: building default configuration returned 2

For more information: try re-running your command with --verbose or --debug

where ${SRC_DIR} is the location of th used viewer source checkout.

Regression

Before autobuild, it was possible to produce 64bit viewers (though only standalone).