cmd/ld: correct pe section names if longer then 8 chars

gcc 4.9.1 generates pe sections with names longer then 8 charters.

From IMAGE_SECTION_HEADER definition:

Name
An 8-byte, null-padded UTF-8 string. There is no terminating null character
if the string is exactly eight characters long. For longer names, this
member contains a forward slash (/) followed by an ASCII representation
of a decimal number that is an offset into the string table.

Our current pe object file reader does not read string table when section
names starts with /. Do that, so (issue 8811 example)

c:\go\path\src\isssue8811>go build
# isssue8811
isssue8811/glfw(.text): isssue8811/glfw(/76): not defined
isssue8811/glfw(.text): undefined: isssue8811/glfw(/76)

becomes

c:\go\path\src\isssue8811>go build
# isssue8811
isssue8811/glfw(.text): isssue8811/glfw(.rdata$.refptr._glfwInitialized): not defined
isssue8811/glfw(.text): undefined: isssue8811/glfw(.rdata$.refptr._glfwInitialized)

Small progress to

Update issue 8811

LGTM=iant, jfrederich
R=golang-codereviews, iant, jfrederich
CC=golang-codereviews
https://codereview.appspot.com/154210044