boot: Fix alignment of long long inside structs on x86

On x86 EFI follows the windows ABI, which expects 8-byte aligned long
long. The x86 sysv ELF ABI expects them to be 8-byte aligned when used
alone, but 4-byte aligned when they appear inside of structs:

    struct S {
        int i;
        long long ll;
    };

    // _Static_assert(sizeof(struct S) == 12, "x86 sysv ABI");
    _Static_assert(sizeof(struct S) == 16, "EFI/MS ABI");

To get the behavior we need when building with sysv ELF ABI we need to
pass '-malign-double' to the compiler as done by EDK2.

This in turn will make ubsan unhappy as the stack may not be properly
aligned on entry, so we have to tell the compiler explicitly to re-align
the stack on entry to efi_main.

This fixes loading EFI drivers on x86 that were previously always
rejected as the EFI_LOADED_IMAGE_PROTOCOL had a wrong memory layout.

See also: https://github.com/rhboot/shim/pull/516

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