go: Upgrade to 1.15 major release

go: Upgrade to 1.15 major release

1.15 is latest major release changelog is [1] and detailed blog is [2]

Drop hardcoding ldso patch in favor of setting it using GO_LDSO variable
which can be defined in terms of linuxloader defined by OE

Setting GOBUILDMODE to pie is no longer needed

[1] https://golang.org/doc/go1.15
[2] https://blog.golang.org/go1.15

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>