dashboard/builder: set correct path to gcc
dashboard/builder: set correct path to gcc

Otherwise when running -386 and -amd64 on the same machine
with both copies of TDM-GCC installed (the installer wizard lets
you choose which one, and you can do both options in two
different install attemps), one previously got this error when
using the 64-bit gcc with a 386 build:

--- FAIL: TestStdcallAndCDeclCallbacks (0.12s)
panic: Failed to load C:\Users\WINDOW~1\AppData\Local\Temp\1\TestCDeclCallback960696761\test.dll: %1 is not a valid Win32 application. [recovered]
       panic: Failed to load C:\Users\WINDOW~1\AppData\Local\Temp\1\TestCDeclCallback960696761\test.dll: %1 is not a valid Win32 application.

goroutine 81490 [running]:
runtime.panic(0x5b8c00, 0x1390d4e0)
        c:/gobuilder/windows-386-gce-b24422c8828f/go/src/pkg/runtime/panic.c:130 +0xed fp=0x12d99f78 sp=0x12d99f24
testing.func?006()
        c:/go/src/pkg/testing/testing.go:421 +0x147 fp=0x12d99fd0 sp=0x12d99f78
----- stack segment boundary -----
runtime.panic(0x5b8c00, 0x1390d4e0)
        c:/gobuilder/windows-386-gce-b24422c8828f/go/src/pkg/runtime/panic.c:98 +0x17a fp=0x12155e98 sp=0x12155e44
syscall.MustLoadDLL(0x1390b7c0, 0x4a, 0x2)
        c:/go/src/pkg/syscall/dll_windows.go:62 +0x6c fp=0x12155eb8 sp=0x12155e98
runtime_test.(*cbTest).run(0x12155f24, 0x1213cc00, 0x1390b7c0, 0x4a)
        c:/gobuilder/windows-386-gce-b24422c8828f/go/src/pkg/runtime/syscall_windows_test.go:399 +0x36 fp=0x12155ef4 sp=0x12155eb8
runtime_test.TestStdcallAndCDeclCallbacks(0x1213cc00)
        c:/gobuilder/windows-386-gce-b24422c8828f/go/src/pkg/runtime/syscall_windows_test.go:448 +0x28e fp=0x12155fac sp=0x12155ef4

TBR=cmang
R=cmang
CC=adg, golang-codereviews
https://codereview.appspot.com/141070043

webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools