main: Rewrite goto statement for Clang

Clang 4.0.0-+rc1-1 and earlier fail to build Memtest86+ with the error
below.

```
$ make CC=clang-4.0
[…]
clang-4.0 -Wall -march=i486 -m32 -O0 -fomit-frame-pointer -fno-builtin -ffreestanding -fPIC  -fno-stack-protector -fgnu89-inline   -c -o main.o main.c
main.c:223:2: error: indirect goto in function with no address-of-label expressions
        goto *ja;
        ^
1 error generated.
```

Fix it, as suggested in the LLVM/Clang bug report #18658 [1].

[1] https://llvm.org/bugs/show_bug.cgi?id=18658#c4
    "Bug 18658 - fatal error: indirect goto in function with no address-of-label expressions"

Change-Id: I4a999c9937daa146f56516bc342138e1c600a1be
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/18253
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed